My experience w/my '90 740 is that when the "check engine" light comes on, it stays on until codes are read and the stored fault is cleared by user intervention.

We just finished a trip that involved at one point some 60 miles of gravel road, half of which were a relatively gentle up-grade, topping out at about 6,000' and due to the nature of the road requiring to be driven at not more than about 20mph. Though it was not particularly warm, for "passenger comfort" we had the A/C on because opening the windows would have sucked a lot of dust into the car.

While doing this part of the drive, the "check engine" light came on. I figured I'd find out later what was up, but when I restarted the car later the light went out as normal.

Later on the same trip, doing some fairly major grades at about 75mph and distinctly hot outside w/A/C running the same sequence happened. "CE" comes on, is off at next restart.

There was no noticeable misbehavior on the part of the car when the CE lamp came on. No codes appeared when I ran the little diagnostic LED through its paces. The car is well maintained and in good health.

Anybody here know what this might have signified (other than an old car saying "give me a break, already")?